河床中值粒径对桥墩冲刷坑演化过程的影响

Numerical Study on the Influence of Riverbed Median Particle Size on Scour Pit Evolution Process around Bridge Pier

桥墩的局部冲刷导致河床形态变化和桥墩基础埋深减小是桥梁水毁的主要原因。在大涡模拟(Large Eddy Simulation,LES)的基础上结合水流运动方程和泥沙运动的动理学理论系统地对桥墩基础处的水流冲刷问题进行全时段全方位的三维数值模拟。得到了桥墩基础处的湍流流场流线图及河床形态变化的高程图。重点研究了水流流速和河床颗粒中值粒径对桥墩周边局部冲刷的影响。结果表明:冲刷坑的深度随着初始流速的增大而增加,且冲刷坑形成速度加快;冲刷坑的深度随着河床颗粒中值粒径的减小而增大,但是当颗粒的中值粒径小到一定程度时,由于泥沙颗粒之间的黏聚力增大导致冲刷坑的深度反而减小。

The change in riverbed morphology and the reduction of bridge pier foundation’s buried depth resulted from local scour around the pier are major causes of bridge failure.An all-time and all-round 3D numerical simulation on the flow scour around the foundation of bridge pier is carried out by using LES(Large Eddy Simulation)in association with the equation of flow motion and the theory of sediment dynamics.The streamlines of eddy flow around the bridge pier foundation and the contours of riverbed elevation were acquired.The effects of flow velocity and median particle size on local scour around bridge pier were examined.The numerical simulation results suggest that the depth of the scour pit increases with the increase of the initial flow velocity and the formation of scour pit accelerates in the meantime.Scour pit depth also extends with the reduction of riverbed’s median particle size;but when the median particle size is small to a certain extent,the depth of scour pit shrinks because of the increasing of cohesion between sediment particles.